Clamp the other 2 horizontal members in place.

Align the other horizontal members with the ones below the jig, so that they are both at the same location relative to the vertical members.

This is crucial so that the machine will stand up straight and there won't be any gap between the drawer and the top horizontal members.

Insure once more that they are square with the vertical members.

Mag Drill them.

Remove the clamps and the jig from the frame.

Bolt the horizontals tightly to the frame.